The config object is quite heavyweight, so we should move this instead
of copying.

We can use contains() here, and also move the mod config if it's valid
instead of copying it, since it contains quite a bit of allocated data.

Allows using keys that aren't directly std::string as the key. This lets
us use std::string_view for the incoming path name, making it more
flexible with other string types.

Allows containers to optimize off of std::move_if_noexcept

We can emplace and move to avoid doing object copies.